Bandpass filters are two-port networks that provide transmission at frequencies within the passband of the filter and attenuation of other frequencies outside of the band. Microstrip bandpass filters are small in size and easy to fabricate. Microstrip coupled line bandpass filters comprise a cascade of parallel half-wavelength-long printed resonators open-circuited at both ends. The resonators are positioned parallel to each other, in such a way that adjacent resonators are coupled along the length equal to the guided quarter-wavelength of the center frequency of the filter.
Disadvantages of conventional microstrip bandpass filters include the existence of a spurious mode which occurs at nearly twice the passband frequency. The spurious mode is caused by the different even-mode and odd-mode propagation velocities of the coupled microstrip resonators. Other disadvantages of these conventional microstrip filters are radiation from open ends, and difficulty obtaining a narrow passband. Also, as with many microelectronic circuit components, the required physical size of conventional microstrip bandpass filters is a limitation to circuit miniaturization.